Conventional public switched telephone systems utilize geosynchronous satellite networks to relay a small portion of long distance telephone traffic. Some of the radio signals that convey these telephone calls are grouped into sequences of digital bits called "packets." In conventional packet networks are carried to their destination through a pathway called a "virtual circuit." A virtual circuit is a route through a network from an origin to a destination that is established at the initiation of a call, and then is released when the call is terminated. The virtual circuit defines a path that includes nodes and links in the network that are used by all the packets associated with the virtual circuit. The packets arrive at their destinations by following instructions or maps that are generated at the origin of the call. Any change in network topology after the call is initiated and the packet itinerary is generated, such as the failure of a node or link, causes a discontinuity in the virtual circuit. When the virtual circuit is disturbed, the packet's itinerary is no longer valid, and the call is disconnected. Re-establishing a broken call by generating a new virtual circuit every time a failure occurs is a lengthy, inefficient and expensive process.
Conventional systems that employ packets that are switched through virtual circuits are capable of providing reliable communications service in networks that are static. If the nodes and the links connecting the nodes are dynamic, the use of packets that follow directions along a fixed itinerary in a virtual circuit that was mapped at the origin of the call becomes impractical. A satellite communications system that utilizes satellites in geosynchronous orbit is generally a static system, since the spacecraft do not change position appreciably relative to relay stations on the ground beneath them. Geosynchronous satellites, however, operate in very high 22,000 mile orbits, and the propagation delays inherent in conveying signals over a nearly 50,000 mile round-trip are not acceptable for many telephone environments.
This intolerably long transit delay is only one of several reasons that militate against the use of high Earth orbit geostationary satellites for a world-wide communications system. One alternative to a geosynchronous system is a constellation of satellites operating in low Earth orbits. In the past, this alternative was not viable, because conventional switching methods that employ packets routed through static virtual circuits using itineraries generated at the origin of a call can not cope with the rapidly changing topology of a low Earth orbit network. No single public communications network is presently capable of offering continuous global service to a wide variety of customers using mobile, portable, and fixed terminals. The problem of providing an economically viable, 24-hour, world-wide network for voice, data, and video has presented a major challenge to the communications business. The development of a fast packet switch which could direct packetized call traffic through a constantly changing network would constitute a major technological advance and would satisfy a long felt need within the electronics and telephone industries.